The present invention relates to a MIG (Metal Inert Gas) process for the electric-arc welding of aluminium and aluminium alloys in pulsed mode or in spray mode, with the exception of a process in spray mode with modulated current.
The electric-arc welding process using a meltable wire as electrode and an inert gas for shielding the welded zone is well known and widely used in industry. Usually, it is called the MIG welding process.
When the meltable wire, that is to say the electrode, melts, the molten metal can be transferred according to several modes. Thus, mention may be made of the modes:
called xe2x80x9cshort-circuitingxe2x80x9d mode, in which a drop of molten metal forms, at a low arc energy, at the end of the meltable wire, then grows and thereafter comes into contact with the weld puddle before detaching from the wire;
called xe2x80x9csprayxe2x80x9d mode, in which the transfer of metal from the wire to the puddle of molten metal takes place, at high energy and high current density, and at a high speed, in the form of fine droplets having a diameter of less than that of the wire and along the axis of the said wire. It is characterized by a very stable arc and no spatter of metal;
called xe2x80x9cpulsedxe2x80x9d mode, in which current pulses are superimposed on a base current intended to maintain the arc, so as to send one drop of molten metal per current pulse. This mode is comparable to the spray mode (with axial spraying), but with the use of a lower mean current.
In order to follow the technological developments in industry, such as those of the transportation (railway, naval, automobile, aeronautic or space) sector or the energy and storage sector, the MIG process has had to be adapted.
First of all, there has been significant development in current sources (power electronics with microprocessor control) and this has been followed by a more rational choice of welding gases, which has allowed the expectations of industry to be more specifically met, particularly in terms of welding productivity and quality.
Thus, it has been shown that argon leads, both in automatic and manual welding, to good transfer of metal into the arc and a beautiful shiny appearance of the welds thus produced.
On the other hand, argon does not allow high welding rates and can give rise, in automatic welding, to arc instabilities above 500 A.
In addition, the narrow characteristic shape of the penetrations obtained under argon do not fit well with welding by interpenetration.
In mixtures of argon and helium (hereafter called Ar/He mixtures), the helium makes it possible to increase the depth of penetration and its root width, and therefore is able to allow expensive preparations to be dispensed with, this being all the more pronounced the higher the helium content.
In other words, for a constant thickness, it therefore allows higher welding rates the higher its content.
In general, the compactness of the beads is also improved, but to the detriment of the appearance of the beads, which are less shiny than under pure argon.
The Ar/He mixtures are therefore of obvious advantage in terms of quality and productivity both in manual welding (Ar+20% He) and in automatic welding (Ar+50% to 70% He), but faced, however, with a not insignificant cost owing to the helium content.
For applications not necessarily requiring these two criteria to be met, it may be judicious to consider other types of mixtures.
Thus, document EP-A-639,423 proposes, for TIG and MIG processes, the use of a welding gas of the argon or argon/helium type containing, in addition, from 100 to 1000 ppmv of CO2 and/or O2.
Furthermore, document DE-A-4,241,982 proposes the use of argon or an argon/helium mixture to which, moreover, from 80 to 250 ppmv of nitrogen has been added.
However, none of these known processes is entirely satisfactory from the industrial standpoint.
What is more, in modulated spray MIG welding, that is to say with the welding current being modulated, it has already been recommended to use a shielding gas or gas mixture, formed from argon, helium or their mixtures, to which from 0.01% to 1.80% of carbon dioxide and/or oxygen has been added, as described in EP-A-909,604.
However, in this case, current modulations at a frequency of less than 60 Hz are applied to the current so as to be able to outgas the weld puddle in order to remove gaseous inclusions therefrom, particularly diffusible hydrogen, which are liable to be found therein.
Thus, MIG processes in spray mode with current modulations are employed when it is desired to obtain a high quality of the welded joint, but without the real need to achieve a high welding speed.
Consequently, the problem that arises is to improve the known MIG welding processes that do not involve modulating the welding current, particularly MIG processes in unmodulated spray mode, that is to say without modulating the welding current, as well as those in pulsed mode, in order to obtain high performance in terms of welding productivity and speed.
To do this, MIG processes in unmodulated spray mode and those in pulsed mode are much more suitable when it is desired to improve the productivity rather than the quality, that is to say the appearance, of the welds thus produced.
However, hitherto MIG processes in unmodulated mode or in pulsed mode have been used little, or not at all, for welding aluminium or its alloys when the gas shield contains oxygen.
This is because it is commonly recognized that the presence of oxygen in the gas shield may have a deleterious impact on the weld given that, when oxygen is incorporated into the gas shield, it can easily combine with aluminium atoms and result in solid inclusions of alumina (Al2O3) in the weld, these having a negative impact on the mechanical properties of the said weld. This has, moreover, been confirmed in the case of high oxygen contents, that is to say oxygen contents greater than 2%, as well as in the case of high carbon dioxide contents, that is to say, again, contents greater than 2%.
However, conversely, the presence of oxygen in the stream of shielding gas results in acceptable productivity levels.
It therefore follows that the problem which arises is to provide an MIG welding process for aluminium and its alloys which makes it possible to achieve both a high and industrially acceptable productivity and a low concentration of alumina inclusions in the weld without a major or appreciable impact on the mechanical properties of the welded
The solution provided by the present invention therefore relies on a process for the MIG welding of aluminium and aluminium alloys in spray mode without current modulation or in pulsed mode, with the use of gas shielding of at least part of the welding zone, characterized in that the gas shield is a gas mixture consisting of 0.01% to 1.80% oxygen and of 98.20% to 99.99% argon.
Further features of the process of the invention are given below:
the shielding gas mixture contains from 0.5 to 1.8% oxygen, the balance being argon;
the gas mixture or shield contains from 1 to 1.7% oxygen, the balance being argon;
the gas mixture or shield contains from 1.2 to 1.65% oxygen, the balance being argon;
a solid meltable wire or a flux-cored wire is used;
the welding speed is from 0.25 m/minute to 1.20 m/minute, preferably from 0.60 to 1 m/minute;
the wire feed rate is from 2.5 m/minute to 20 m/minute, preferably from 4 m/minute to 13 m/minute;
the mean welding current is from 40 A to 450 A and/or the mean welding voltage is from 15 V to 40 V;
the process is in pulsed mode and/or the welding current is from 120 A to 350 A and/or the mean welding voltage is from 20 V to 28 V;
the process is in spray mode and/or the welding current is from 180 A to 450 A and/or the mean welding voltage is from 20 V to 39 V.
The present invention therefore relies on precise control of the oxygen content in the argon, the oxygen content having not to exceed approximately 1.80% at most; the gas mixture thus formed constitutes the gas shield used during the use of the MIG process.
It should be emphasized that any MIG process in spray mode with modulation of the welding current is excluded from the invention.